1. Field of Invention
This invention relates to a halogen-free resin composition containing metal hydroxide, to a method of making the resin composition and to electrical wire having this resin composition as a covering on an electrical conductor core. Such an electrical wire is useful, for example, in a motor vehicle.
2. Description of Related Art
Polyvinyl chloride has been widely used as the covering material of electrical wire for an automobile, because it is superior in properties such as mechanical strength, extrusion processability, flexibility and coloring property. However, with recent concern for the global environment, halogen-free resin material has come to be used for the production of automobile parts including the covering of electrical wires in an automobile in place of polyvinyl chloride, because polyvinyl chloride discharges a harmful halogen gas on combustion.
A halogen-free resin composition in which a metal hydroxide is blended with a polyolefin-base polymer as a flame-retardant is known as a wear resistant resin composition having the merit of no generation of a poisonous gas such as a halogen gas on combustion (see JP-A-7-176219, JP-A-7-78518 and the like). In order that such a flame-retarding resin composition has a self-extinction property, a large quantity of a metal hydroxide is required to be added; however, this causes problems that mechanical strength such as the wear resistance, tensile strength and the like of the composition are much reduced. In order to prevent the deterioration of mechanical strength, it may be considered that amounts of a polypropylene having a comparatively high hardness and a high density polyethylene are increased, but the flexibility of the covered electrical wire is reduced thereby and the processability becomes poor.
Various specific prior art proposals in this field will now be mentioned.
JP-A-6-290638 discloses resin compositions containing metal hydroxide for electrical wire insulation, in which the resin composition is based on polypropylene (>80%). Additional components are polyethylene modified with acid anhydride and styrene copolymer.
U.S. Pat. No. 5,561,185 describes resin composition for electrical wires containing metal hydroxide, in which the resin components are (a) 40-88.5% by weight of propylene which is 50% by weight or more of a ethylene/propylene random copolymer, (b) 1.5 to 30% by weight of a polyethylene modified with carboxylic acid derivative, e.g. maleic anhydride and (c) 10 to 48% by weight of an ethylene-series copolymer, typically ethylene/vinyl acetate copolymer.
U.S. Pat. No. 5,180,889 also describes a resin composition containing metal hydroxide as a covering of conductors in a crush resistant cable assembly. The resin components are (a) a low density copolymer of ethylene and alpha-olefin, (b) an elastomeric styrene-ethylene-butylene-styrene tri-block copolymer, preferably modified with maleic anhydride and (c) optionally an impact propylene and copolymer or polypropylene. Component (a) in the examples is 50% by weight or more of the total resin components.